4-Amino-3-p-halophenylbutyric acids and suitable derivatives thereof are disclosed in U.S. Pat. Nos. 3,471,548 and 3,634,428 and one member thereof, i.e. baclofen [4-amino-3-p-chlorophenylbutyric acid or .beta.-(aminomethyl)-p-chlorohydrocinnamic acid respectively], is the active ingredient of the muscle relaxant LIORESAL.RTM.. Moreover, non-spasmolytic doses of baclofen were described in Brit. J. Pharmacol. 54, 171 (1975) as enhancing the analgesic action of morphine. Surprisingly it was found that said 4-amino-3-p-halophenylbutyric acids and derivatives thereof, especially baclofen, do not necessarily enhance the other effects of morphine, in particular the drug seeking behavior and physical dependence liability. They actually depress the symptoms of withdrawal of addicting agents, particularly of narcotics, such as morphine, and reduce the craving for said agents, particularly for morphine administration. Therefore, combinations of addicting agents, e.g. narcotics, especially with said 4-amino-3-p-halophenylbutyric acids, or derivatives thereof, should be administered, in order to prevent future addiction, or to ameliorate the withdrawal symptoms in the addicted.